GEOLOGIC TIME


Debates About the Age of the Earth


	Bishop Usher



	Lord Kelvin



	Discovery of Radioactive Decay




Most geologists believe that the Earth formed 
about 4.6 billion years ago



	The continental crust of the Earth is ~4.0 b.y.


	The oceanic crust is much younger

Relative Time



Order events from oldest to youngest              
Principle of Superposition





Principle of Original Horizontality



Principle of Lateral Continuity



Principle of Cross-Cutting Relationships



Principle of Inclusions



Principle of Faunal Succession




Geologic Time Scale



Absolute Time



Methods relying on event in the geological 
record with very strong annual cyclicity: 



tree growth rings 



coral growth cycles 



varves (annual clay sediment layers)




Methods relying on the decay of naturally 
occurring radiogenic isotopes



Half-life : time it takes for half of the Parent 
to decay (change) to the Daughter


	Uranium-235 -> Lead-207 


	Uranium-238 -> Lead-206 


	Thorium-232 -> Lead-208 


	Rubidium-87 -> Strontium-87


	Potassium-40 -> Argon-40

	Carbon-14 -> Nitrogen-14

Decay Schemes

During the decay Heat is given off - this is an 
important source of energy to produce 
temperatures necessary for partial melting.
alpha decay - two protons and two neutrons are 
emitted from the nucleus. This reduces the 
atomic number of the parent by 2 and the 
mass number of the parent by 4. Uranium to 
Lead Schemes

Electron capture - occurs when a proton 
captures an electron and changes into a 
neutron. The atomic number of the parent 
element is decreased by 1 but the mass 
number is unchanged : 


Beta decay - an electron is emitted from a 
neutron in the nucleus changing the neutron 
to a proton. This increases the atomic number 
of the parent element by 1 but does not 
change the atomic mass number :

Parent 		 Daughter 		Half-Life


C-14		       N-14	         5,730


U-235   		Pb-207	       710,000,000                                                           
 
K-40			A-40	     1,300,000,000
                                                                                                      
U-238		Pb-206	     4,500,000,000


Th-232		Pb-208	    15,000,000,000


Rb-87		Sr-87	   47,000,000,000


Implications for heat flow due to radioactive 
decay during Earth history





Geologic Time Scale